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SavannahYear 8
← Computing
ComputingGCSE computer science: algorithms and programming30 min★★★ difficulty

Algorithms, program constructs and testing

Paper 2 rewards precision. This lesson covers the searches and sorts you must know, the three constructs, and how to design test data that finds bugs.

Lesson overview

What you'll learn in this lesson

Design algorithms, write programs using core constructs, and test and debug them.

Key learning points

  • • Searching
  • • Sorting
  • • The three constructs
  • • Testing and error types

This lesson at a glance

  • 30 minutes
  • 16 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

designalgorithmsprogramsconstructssearching

Scroll down — the lesson carries on below

Part 1 of 16 · Discover6%
1

Watch & discover

Part 1 of 16

Watch first

Algorithms, program constructs and testing

An English teaching video on this topic from Mr J Online. Watch it, then scroll down for the lesson.

In a nutshell

Design algorithms, write programs using core constructs, and test and debug them.

2

Learning cycle

Part 2 of 16

Learning cycle 1 of 2

Part 1 · Searching

A short piece of teaching, then a check to make sure it has landed.

3

Explore the idea

Part 3 of 16

Learn

Searching

A linear search checks each item in turn: it works on unsorted data and takes on average half the list. A binary search repeatedly halves a sorted list by comparing with the middle item: it is far faster on large data sets but requires the data to be sorted first.

4

Explore the idea

Part 4 of 16

Learn

Sorting

Bubble sort repeatedly swaps adjacent out-of-order items — simple but slow. Merge sort divides the list until each part holds one item, then merges pairs in order — consistently efficient. Insertion sort builds a sorted section one item at a time and is quick on nearly sorted data.

5

Quick check

Part 5 of 16

Quick check

What must be true before a binary search?

6

Quick check

Part 6 of 16

Quick check

Which sort is quickest on nearly sorted data?

7

Reset break

Part 7 of 16

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

Stop here for now
8

Learning cycle

Part 8 of 16

Learning cycle 2 of 2

Part 2 · The three constructs

A short piece of teaching, then a check to make sure it has landed.

9

Explore the idea

Part 9 of 16

Learn

The three constructs

Sequence runs statements in order; selection chooses a path with if, else if and else; iteration repeats with a count-controlled for loop or a condition-controlled while loop. Combine these with arrays, records, subprograms and file handling and you can write any GCSE program.

10

Explore the idea

Part 10 of 16

Learn

Testing and error types

Syntax errors break the rules of the language and stop the program running; logic errors run but give the wrong result. Test data must be normal, boundary and erroneous — and boundary values are where the marks and the real bugs are.

11

Quick check

Part 11 of 16

Quick check

Which error allows the program to run but gives a wrong answer?

12

Quick check

Part 12 of 16

Quick check

Which test data most often exposes an off-by-one mistake?

13

Explore the idea

Part 13 of 16

Worked example

Worked example: validating an age input

A program must accept an age between 11 and 18. Using a while loop, the program repeatedly asks for input while the value is outside the range, so invalid entries cannot pass. Test data should include 15 as normal data, 11 and 18 as boundary data that must be accepted, 10 and 19 as boundary data that must be rejected, and 'twelve' as erroneous data that must be handled without crashing. Testing only normal data would let an off-by-one comparison such as age > 11 pass unnoticed, which is exactly why boundary values are specified.

14

Challenge round

Part 14 of 16

Game · Sort it

Which of these are true?

Drag each card into the right column. Tap a card first if dragging is fiddly.

True

Not true

15

Mastery quiz

Part 15 of 16

Marked quiz

End of lesson quiz: Algorithms, program constructs and testing

4 questions, marked with the reasoning shown. No timer.

  1. 1. What must be true before a binary search?

  2. 2. Which sort is quickest on nearly sorted data?

  3. 3. Which error allows the program to run but gives a wrong answer?

  4. 4. Which test data most often exposes an off-by-one mistake?

16

Lesson round-up

Part 16 of 16

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

0

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Article

How a supermarket checkout knows the price

When a barcode is scanned, the checkout does not store the price itself. It sends the barcode number to a database and reads back a record: product name, price, whether it needs an age check. The record is the single place the price lives, so changing one row changes every till in the shop at once.

Problems come from bad data, not bad scanners. If two products share a barcode, or a price update reaches the shop overnight but the till holds an old copy in its cache, customers are charged the wrong amount. Shops check this with test transactions: scan a known item, expect a known price, record what actually happened.

Good systems also log every scan. If a customer complains, the log shows exactly which record was used and when.

Written for Good Learning Co.

Reading questions

  1. Write in your own words what the checkout sends and what it receives back.
  2. Explain one way bad data could cause a wrong price, and how logging helps find it.
  3. Design a test table with four tests for a new checkout, with expected and actual results.

  • Reading and notes

    Reading

    Read the lesson text again and note the key terms and how each one is used, in your own words — as you would for revision.

    Half a page of notes

  • Extended writing

    Writing

    Explain a real system that uses this idea — how it works, where it can go wrong, and how you would make it safer or better.

    250–350 words

  • Algorithm on paper

    Diagram

    Write your steps out as pseudocode or a flowchart before you type anything, then photograph it.

    1 flowchart or 10 lines

  • Code write-up

    Writing

    Copy your code in, then explain underneath what each part does and what you had to fix.

    Code + 5 sentences

  • Test table

    Working out

    Make a test table: what you tried, what you expected, what actually happened.

    4–6 tests

Saved as you type.

Part 1 of 16 · Watch & discover

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